The present invention relates to a method and apparatus which is used to retain spinal elements in a spatial relationship. More particularly, a modular pedicle screw assembly of the invention consists of a base fastener and connecting headpieces which are varied in length and angularity in order to provide a clinician a greater degree of flexibility in stabilizing the spine when performing spinal fusions in the cervical, thoracic, and lumbar regions.
Important considerations for spinal instrumentation used to promote spinal fusion are safe insertion, rigid fixation, and ease and adaptability of implantation. Historically, Harrington rods were an early type of spinal instrumentation used to stabilize the thoracic and the upper lumbar regions to obtain correction and stability for fusion. This instrumentation involved long rods attached to hooks, which were inserted over the bony arches (lamina) of the spinal canal. The instrumentation was subject to failure when the hooks either broke the lamina or became displaced and thus, the intended stabilization and correction was lost.
Subsequent spinal instrumentation included the use of a longitudinal member, such as a plate, to interconnect a series of pedicle screws such as that disclosed in U.S. Pat. No. 5,129,899 issued to Laura C. Small ("Small, et al"). In this system, the clinician must strive to place the series of pedicle screws into the bone in longitudinal alignment with as little lateral displacement and angular deflection as possible so that the plate may be attached to the series of pedicle screws in order to achieve a solid biomechanical relationship. However, due to variances in the spinal column or imperfect placement by the clinician, the screws are often inserted at an angle or must be placed laterally outside the longitudinal axis defined by the plate and other screws. While slight angular deviations of screw placement of up to 20.degree. may be overcome by the use of contoured washers, larger angular deflections and the problems associated with lateral displacement of the pedicle screws cannot be overcome by the instrumentation disclosed in the Small, et al. patent.
The development of the Kambin Offset Bolt for the use with the device disclosed in the Small, et al. patent allows greater flexibility when the pedicle screws cannot be placed in the desired longitudinal relationship. The Kambin system, which is offered by Smith & Nephew and known to those of skill in the art, uses bolts having lateral extensions that allow attached threaded shanks to reach the plates in a more desirable biomechanical relationship. The offset bolt, however, has several drawbacks. First, insertion of the offset bolt requires a larger area of clearance to obtain insertion. This often leads to insertion difficulty when the bolt interferes with the spinal retractors, other elements of the spine, or other screws which have already been implanted. This interference can cause other implanted screws to loosen when impinged. In addition, once in place, the offset bolt can only overcome translational malalignment, but not angular malalignment of the pedicle screws.
Another system is disclosed in U.S. Pat. No. 5,129,900 ("Asher, et al."). There, pedicle screws are connected to a longitudinal member, such as a rod. Lateral deviations may be cured by the use of connector members which include oblong openings. (Col. 4 lines 54-67). Like the offset screw taught by the Small et al. patent, however, the device disclosed in the Asher, et al. patent cannot overcome the difficulties associated with angular malalignment.